Ferrite devices are used extensively to control the propagation of high frequency electromagnetic wave energy, particularly energy in the microwave and millimeter wave spectrums. These devices include a wave transmission system, which may use waveguide, stripline, microstrip, or coaxial transmission line technology, and magnetically biased ferrite material located in or adjacent to the wave transmission system.
Typically, the ferrite material is magnetically biased by applying a constant magnetic field whose direction and magnitude are selected to produce a desired attenuation, phase shift, or diversion of electromagnetic wave energy propagating in the wave transmission system. Presently, ferrite devices use permanent magnets or electromagnets requiring power supplies to provide the required constant magnetic field.
The size and weight of permanent magnets or electromagnets add significantly to the size and weight of ferrite devices. In fact, such magnets often weigh several times more than the combined weight of all the other components in the ferrite device. The added size and weight are particularly significant at higher microwave and millimeter wave frequencies since the strength of the constant magnetic field and, therefore, the size and weight of the magnet required for proper operation of the ferrite device increase as the frequency of electromagnetic wave energy propagating in the wave transmission system increases.
The term "ferrite device" as used in this application refers to any device which relies on the interaction of electromagnetic wave energy propagating in a wave transmission system with magnetically biased ferrite material to control the wave energy in a desired manner.
Well known ferrite devices include circulators, isolators, attenuators, switches, modulators, yttrium iron garnet (YIG) filters, and phase shifters, among others.
While the present invention is applicable to all such ferrite devices, the preferred embodiment will be described as applied to a circulator, one of the most widely used ferrite devices.
A conventional microstrip junction circulator 2 of the type commonly used in transmit/receive systems for duplexing a transmitter and receiver to a common antenna is shown in FIGS. 1a and 1b, in exploded and perspective views, respectively. Microstrip junction circulator 2 is designed to provide an impedance match between the antenna and the transmitter and receiver and to protect sensitive circuitry in the receiver from high power produced by the transmitter while the system is transmitting.
Microstrip junction circulator 2 includes a ground plane 5 and a microstrip conductor 3 having a junction 4 which is connected to radially extending transmission lines 6, 8, and 10. A ferrite disk 14 is located between junction 4 and ground plane 5. Transmission lines 6, 8, and 10 are connected to center conductor pins 13 of coaxial connectors 7, 9, and 11, respectively. Ground plane 5 has a notch 15 to accept permanent magnet 12. Permanent magnet 12 produces a constant magnetic field which biases ferrite disk 14 and is substantially parallel to the axis of ferrite disk 14.
As is well known, by magnetically biasing ferrite disk 14 of circulator 2 with a constant magnetic field which is of appropriate strength and substantially parallel to its axis, electromagnetic wave energy received over one transmission line is caused to propagate in a circular direction around ferrite disk 14 to the next transmission line. The direction in which the electromagnetic energy propagates depends on the polarity of the constant magnetic field.
FIG. 1c is a schematic diagram illustrating the operation of circulator 2 in a transmit/receive system. As shown in FIG. 1c, transmission line 6 is connected to a transmitter 16; transmission line 8 is connected to a receiver 18; and transmission line 10 is connected to an antenna 20. The polarity of the constant magnetic field produced by magnet 12 is such that electromagnetic wave energy received by transmission line 10 from antenna 20 is circulated in a counterclockwise direction around ferrite disk 14 to transmission line 8 where it is coupled to receiver 18. Transmission line 6 and transmitter 16 are thereby isolated from electromagnetic wave energy received by transmission line 10 from antenna 20.
The constant magnetic field produced by magnet 12 will also cause electromagnetic wave energy received by transmission line 6 from transmitter 16 to circulate in a counterclockwise direction around ferrite disk 14 to transmission line 10 where it is coupled to antenna 20. Transmission line 8 and receiver 18 are thereby isolated from high power electromagnetic wave energy produced by transmitter 16.
FIGS. 2a and 2b show exploded and side views, respectively, of a conventional stripline circulator 2' which operates in the same manner as the microstrip junction circulator 2 of FIG. 1a and 1b. Stripline circulator 2' has a stripline center conductor 3 located between a lower ground plane 5 and an upper ground plane 5'. Stripline center conductor 3 has a junction 4 which is connected to radially extending transmission lines 6, 8, and 10. Ferrite disk 14 is mounted between junction 4 and lower ground plane 5. Ferrite disk 14' is mounted between junction 4 and upper ground plane 5'. Transmission lines 6, 8, and 10 are connected to center conductor pins 13 of coaxial connectors 7, 9, and 11, respectively. Permanent magnet 12' produces a constant magnetic field which biases ferrite disks 4 and 14' as required for circulator operation.
Circulators of the type shown in FIGS. 1a, 1b, 2a, and 2b typically provide 15 to 20 dB of isolation over a specified bandwidth. Greater isolation can be achieved by cascading circulators together in a known manner.
A current state-of-art use for such circulators is in hybrid microwave integrated circuits (MICs) such as the T/R (transmit/receive) module 1 for a phased array radar shown in FIG. 3. A large multiple of such modules are used in the radar. Transmitter 16', receiver 18', phase shifter 21 and control circuit 17 are individual monolithic microwave integrated circuits (MMICs) which can be manufactured inexpensively and repetitiously by MMIC foundries.
One of the advantages of this arrangement is that a microstrip version of a circulator 2 can be integrated in the module substrate 99 without requiring connectors. Components are integrated via interconnecting microstrip transmission lines 6, 8 and 10. The individual circuits are powered and controlled by integrated conductors 19. The transmission line 10' at one end of the module feeds an antenna element (not shown) in the array. The transmission lines 6' and 8' and DC and control wires 19 at the other end of the module are connected to combiners and distribution networks (not shown) essential to proper operation of the radar. In current radars, the weight of the module 1 is frequently dominated by that of the magnet required by the circulator 2, thereby limiting benefits made possible by hybrid MIC and MMIC technology.
One known method of avoiding the weight penalty of circulator magnets is to use a solid state switching system at each antenna element, instead of a circulator, to isolate the transmitter and receiver. However, unlike circulators, solid state switching systems cannot preserve a constant impedance match to the transmitter and receiver as a function of scan angle. They are also more lossy than circulators and require logic circuitry which adds to system complexity. The added complexity results in reduced mean time to failure and hence reduced system reliability. As a result, where constant impedance, reliability, and energy loss are critical, circulators should be used despite the weight penalty of the circulator magnets.
In addition, there are certain applications, such as in fm or cw communication systems, where solid state switching systems cannot be used. In such applications, circulators are required and the weight penalty introduced by circulator magnets is unavoidable.
Accordingly, a need exists for a constant magnetic field source for circulators and other ferrite devices which is lightweight and compact.